short communications J. Synchrotron Rad. (2020). 27, 503–506 https://doi.org/10.1107/S1600577519016643 503 Received 24 July 2019 Accepted 11 December 2019 Edited by P. A. Pianetta, SLAC National Accelerator Laboratory, USA Keywords: beam filtering; breast computed tomography; Gaussian beam. Flattening filter for Gaussian-shaped monochro- matic X-ray beams: an application to breast computed tomography Sandro Donato, a,b Fulvia Arfelli, a,b Luca Brombal, a,b * Renata Longo, a,b Andrea Pinto, c Luigi Rigon a,b and Diego Dreossi d a Department of Physics, University of Trieste, 34127 Trieste, Italy, b INFN Division of Trieste, 34127 Trieste, Italy, c Department of Medicine, Surgery and Health Sciences, University of Trieste, 34127 Trieste, Italy, and d Elettra-Sincrotrone Trieste SCpA, 34149 Trieste, Italy. *Correspondence e-mail: luca.brombal@ts.infn.it The vertical intensity distribution of synchrotron-based X-ray beams usually has a Gaussian profile encompassing large intensity variations. For biomedical imaging applications this may entail sub-optimal dose distributions and large fluctuations in terms of image noise. Commonly, planar metallic filters coupled with absorbing slits systems are applied to adjust the delivered flux and to limit intensity variations, respectively. The latter results in a reduction of the effective beam size. A flattening filter that counterbalances the transverse inhomogeneity, while retaining a sufficient flux, has been developed in the context of a monochromatic phase-contrast breast computed tomography application, ongoing at the Elettra synchrotron facility. The implementation of the new filtration system results in homogeneous intensity (hence dose) distribution and signal-to-noise ratio across the imaged volume. Finally, and most importantly, it allows a wider portion of the beam to be used, directly translating into a major (40%) reduction of the overall scan time for samples requiring a field of view larger than the beam size (i.e. multiple translation steps). 1. Introduction Besides the high coherence, X-rays produced by synchrotrons are, in general, several orders of magnitude more intense with respect to conventional sources. For this reason, many biomedical imaging applications require beam filtration to deliver acceptable dose levels within a given exposure time (Bravin et al., 2013; Rigon, 2014). This is usually performed by inserting metallic sheets or slabs that reduce the overall beam intensity without affecting its spatial distribution (or ‘shape’). When considering synchrotron radiation produced by bending magnets, the beam vertical (i.e. orthogonal to the electrons orbit plane) intensity distribution is well described by a Gaussian function (Wiedemann, 2003), that leads to an undesired non-uniform dose distribution on the sample in the vertical direction. In terms of image quality this means that, especially for tomographic applications, the signal-to-noise- ratio (SNR) decreases from the central maximum towards the tails. To limit such non-uniformity, in most cases only the central part of the beam is used for imaging purposes, while the tails are cut out by absorbing (e.g. tungsten) slits. Despite being easy to implement, this approach is not optimal for applications requiring tomographic scans of large samples since the reduction of the beam vertical dimension means an increase in the number of vertical scans required to image the whole volume and, as a consequence, an increase in the overall scan duration. ISSN 1600-5775 # 2020 International Union of Crystallography